Expand and combine like terms. $(9c^2+c^6)(9c^2-c^6)=$
Explanation: We can expand this expression like any product of two binomials. However, this expression has a special form that makes it easier to expand. This is the "difference of squares" form (where $P$ and $Q$ can be any monomial): $(P+Q)(P-Q)=P^2-Q^2$ $\begin{aligned} &\phantom{=}(9c^2+c^6)(9c^2-c^6) \\\\ &=\left(9c^2\right)^2-\left(c^6\right)^2 \\\\ &=81c^4-c^{12} \end{aligned}$